The invention relates to device or for sealing the medullary cavity of a bone when applying bone cement.
In arthroplastic surgery, most implants are anchored in the bony bed with the help of so-called bone cement. This bone cement usually consists of polymethylmethacrylates or related compounds. In order to improve the penetration of the bone cement into the honeycomb structure of the bone marrow, attempts were made to wash the bony bed under pressure and to apply the bone cement under pressure. This technique, which is also called the "high pressurising technique", is mostly used with bone cement of low viscosity.
This technique, however, led to a number of fatal incidents. Both animal experiments and clinical examinations showed that an increase in intramedullary pressure may cause reflex cardiac arrest and give rise to fatal fat and bone marrow embolisms. Furthermore, this method did not succeed in keeping the bony bed clear of blood. Rather, blood flowed into the bony bed in dependence on the blood pressure and blended with the bone cement, thus considerably weakening its material properties. Attempts have already been made to improve the mechanical strength of the bone cement used in clinics by vacuum-mixing it and prepressurising it. It is, however, essential for the strengthened material of the bone cement to be transferred to the bony bed of the patients without any loss of its material properties and without endangering the patients' lives.
Thus a method has been developed with which the bony bed can be kept dry and the cement can be applied in an artefact-free manner, and this without jeopardizing the patient with the temporary increase of the intramedullary pressure. In this method, the bone cement is initially mixed under vacuum and prepressurised and is then applied into the medullary cavity using a bone cement pistol or press as described, for example, in EP-A-170 120 or U.S. Pat. No. 4,671,263. At the same time, a cannulated bone screw is used to apply a vacuum distally. In this way the bone cement is sucked deep into the medullary cavity whilst blood, fat and bone marrow are sucked out of the bone canal at the same time. Once the prosthesis component has been inserted, the vacuum lead is pinched off, whereupon a cannulated bone screw is used to proximally apply a vacuum. This means that the vacuum will remain effective in the metaphysis until the bone cement has hardened. This method is characterised by filling the medullary cavity under vacuum and draining it at the same time. It enables complete and artefact-free application of the bone cement around the prosthesis along with deep penetration of the bone cement into the spongiosa.
In order to maintain the required partial vacuum in the medullary cavity and to prevent the bone cement from seeping out, it is essential to keep the medullary cavity as tightly sealed as possible when applying the bone cement.
This is usually aimed at with a collar placed around the tip of the vessel or cartridge with which the bone cement is applied.